Organic light-emitting diodes (OLEDs), also known as organic electroluminescent (EL) devices, are an emerging display technology. In essence an OLED comprises a thin organic layer or stack of organic layers sandwiched between two electrodes, such that when a voltage is applied visible or other light is emitted. At least one of the electrodes must be transparent to light. For display applications the light must of course be visible to the eye, and therefore at least one of the electrodes must be transparent to visible light.
There are two principal techniques that can be used to deposit the organic layers in an OLED: thermal evaporation and solution processing. Solution processing has the potential to be the lower cost technique due to its potentially greater throughput and ability to handle large substrate sizes. Significant work has been undertaken to develop appropriate materials, particularly polymers. More recently phosphorescent organometallic dendrimers that are luminescent in the solid state have been shown to have great promise as solution processible light-emitting materials in OLEDs (S.-C. Lo, et al Adv. Mater., 2002, 13, 975; J. P. J. Markham, et al Appl. Phys. Lett., 2002,80,2645). Although progress has been made in the development of solution processible OLEDs there is still the need for OLEDs with improved efficiency and lifetime.
Dendrimers are branched macromolecules with a core and attached dendrons (also called dendrites). Dendrons are branched structures comprising branching units and optionally linking units. The generation of a dendron is defined by the number of sets of branching points; see FIG. 1. Dendrons of a higher generation, or order, can be composed of the same structural units (branching and linking units) but have an additional level of branching, i.e. an additional repetition of these branching and linking units. Alternatively higher generations can have an additional level of branching but different branching and linking units at the higher generation. There can be surface groups on the periphery of the dendrons.
Light-emitting dendrimers typically have a luminescent core and in many cases at least partially conjugated dendrons. Further examples of light-emitting dendrimers include those found in P. W. Wang, et al Adv. Mater., 1996, 8, 237; M. Halim, et al Adv. Mater., 1999, 11, 371; A. W. Freeman, et al J. Am. Chem. Soc., 2000, 122, 12385; A. Adronov, et al Chem. Comm. 2000, 1701.; C. C. Kwok, et al Macromolecules, 2001, 34, 6821. Light-emitting dendrimers have the advantage over light-emitting polymers that the light-emitting properties and the processing properties can be independently optimised as the nature of the core, dendrons and surface groups can be independently altered. For example, the emission colour of a dendrimer can be changed by simply changing the core. Such light-emitting dendrimers can be useful in electro-optic devices, particularly OLEDs.
Other physical properties, such as viscosity, may also make dendrimers more easily tailored to the available manufacturing processes than polymers. Organometallic dendrimers have previously been used in OLED applications as a single component in a film (i.e. a neat film) or in a blend with a molecular material or in a blend of more than one dendrimer of different type (i.e. different cores), e.g. J. M. Lupton et al. Adv. Funct. Mater., 2001, 11, 287 and J. P. J. Markham, et al Appl. Phys. Lett., 2002, 80, 2645.
Intermolecular interactions play an important role in the opto-electronic properties of organic light-emitting and transport materials. Close contact and good order can lead to high charge mobilities but can also give rise to reduced emission due to the formation of excited state dimers. In previous work we have shown that intermolecular interactions can be controlled by the generation of the dendrons attached to a dendrimer (J. M. Lupton, et al Phys. Rev. B, 2001, 63, 5206; J. P. J. Markham, et al Appl. Phys. Lett., 2002, 80, 2645). However, we have found that for organometallic dendrimers that contain only one dendron per ligand that generation does not always give adequate control over intermolecular interactions. For example, for the iridium based dendrimers in J. P. J. Markham, et al Appl. Phys. Lett., 2002, 80, 2645 (see FIG. 2), the photoluminescence quantum yield of the second generation, 2, is higher than for the first, 1, but both are less than when the measurement is carried out in dilute solution where dendrimer intermolecular interactions are not present. For iridium dendrimers 1 and 2 the dendrons are attached to one component of the bidentate ligand, namely the phenyl ring, and the facial (fac) isomers are formed. This combination leaves one face of the core unprotected by dendrons allowing potentially detrimental core-core interactions.
There are known dendrimers based on tris ruthenium 2,2′-bipyridine cores in V. Balzani, et al Coord. Chem. Rev. 2001, 291-221, 545. In these dendrimers two dendrons are attached to each 2,2′-bipyridine ligand at the 4 and 4′ positions. However 2,2′-bipyridine is a neutral ligand, and these dendritic complexes have a net positive charge which has to be balanced by an associated counter-ion, typically PF6−. The three bipyridine ligands fill the coordination sphere of Ru, so the counter-ion is not part of the inner coordination sphere of the metal, but is more loosely associated. In OLED applications it is undesirable to have unbound counter-ions, as these may be able to migrate under the influence of the applied field, which could be detrimental to the OLED device stability. The present invention relates to organometallic dendrimers that are neutral, i.e. those in which the ligands directly coordinated/bonded to the metal balance the charge.
We have discovered that these disadvantageous core-core interactions that are detrimental to OLEDs can be overcome by changing the dendrimer structure. One way of doing this is to attach more than one dendron to more than one ligand complexed to the metal cation. For example, for an octahedral fac-iridium (III) complex with 2-phenylpyridine ligands a dendron could be attached to both the phenyl and pyridyl rings and then two or more of these ligands complexed to the metal cation. In this embodiment of the invention, the organometallic dendrimers contain more than one dendron attached to each ligand.
A second way of controlling the intermolecular core-core interactions is by using different isomers. For example, for an octahedral fac-iridium (III) complex with 2-phenylpyridine ligands with dendrons attached to the phenyl rings one face of the core is not protected by the dendron. By using the meridinal (mer) isomer the dendrons are more evenly distributed around the core and as a consequence the core is more protected by the dendrons.
The present invention relates to dendrimers, processes of preparing them and opto-electronic devices, in particular OLEDs, containing them, that solve some of the problems in the prior art. In particular, the invention seeks to overcome the intermolecular interactions that are detrimental for OLED performance.